It has been known to utilize an electrically conductive helical coil spring to provide electrical communication between the one end of a brush and a source of electricity. An example of such a device may be found in U.S. Pat. No. 3,376,444. An example of a brush holder utilizing a constant force spring through which current is flowed to the outer end of the brush may be found in U.S. Pat. No. 3,387,156 with particular reference being made to the embodiment illustrated in FIG. 6 thereof. An example of a brush assembly utilizing a constant force twin coil spring may be found in U.S. Pat. No. 2,695,968, and particularly the embodiment illustrated in FIG. 6 thereof.
Although each of the patented brush assemblies may function satisfactorily for its intended purpose, each has certain limitations. For instance, with respect to the twin coil spring brush assembly, electrical communication between an electricity source and the brush is provided by means of a braided or twisted wire having one end connected to the brush and another end connected to a suitable source of electricity. The disadvantage of this construction is that the wire, or shunt as the wire is called in the art, has a certain stiffness which, when combined with the spring, provides a spring system having a gradient which cannot be predicted with any degree of accuracy. This makes it difficult, if not impossible, for engineers to calculate the design of the twin coil springs. The effect on the spring system occurs whether or not the shunt is connected into the brush itself or is interposed between the underside of the saddle portion of the twin coil spring and the outer end of the brush as has been proposed in the art. A further disadvantage of this arrangement resides in the fact that the current flows the entire length of the brush. This is undesirable because it may cause the brush to heat up in use and, therefore, to wear prematurely under certain operating conditions. Accordingly, an improved brush assembly which overcomes these limitations is highly desirable.